The US Air Force does not operate on the same premise as a for profit airline, hence their requirements are going to be different. The 767 is a very proven and reliable airframe platform that transformed transatlantic aviation. For this and many other roles, the military wants a proven platform with tons of parts, knowledge, personnel, etc already out there. The A330 is a great aircraft, but it is not as prevelent as the 767 is, in the areas the US Air Force requires. Also, the A330 is simply too big for this role, but the closest other airframe Airbus offers is even older than the 767, that being the A300/A310 series.
And while the airframe may have been designed in the 70's, just about everything else is current technology. In fact, the 767 is still a viable enough airframe that Boeing and Aviation Partners offer winglet upgrades. These alone can improve fuel efficiency by 5-7%, and I believe are even standard on new builds.
Aircraft design at this scale simply doesn't translate to the consumer mindset of this years iWidget must be so much better than last years.
Maybe in this case the Air Force procured the product that would best match its needs per dollar?
EDIT: As per your suggestion that splitting this role among multiple airframes would lead to higher reliability, maintenance, whatever, see Southwest, Easyjet, Ryanair, etc. Multiple airframes == multiple parts systems, multiple training, multiple depts, etc. Look at the trend of multirole fighter/attack/bombers vs. how many different aircraft provided these roles during the cold war.
PS: I don't expect you to agree or even understand any of this, given your predisposed position indicated by you cleverly sneeking in "Air Farce" rather than Air Force. You go girl!